EPDM's containing vinyl norbornene (VNB), which is a non-conjugated diene having two polymerizable double bonds, are known from EP843698; EP843702 and EP843701. These polymers have long chain branching (LCB). High levels of LCB improve processability, but may impair physical properties (tear) of final product after conversion of the polymer by extrusion or molding etc. The two double bonds are both capable of polymerization with olefins in the presence of transition metal catalysts.
The prior art describes the benefit of VNB over ethylidene norbornene (ENB). ENB is a non-conjugated diene having one double bond that is copolymerizable using a transition metal catalyst. The other double bond is not so polymerizable and remains available in the final polymer for subsequent reaction, e.g., sulfur curing. The VNB derived EPDM provides improved cure rate and performance in free-radical curing, improved processability from the highly branched structure and requires a low level of diene to provide suitable physical properties in the final product comparable to ENB derived EPDM.
WO99/00434 describes combining ENB, VNB and specific branching inhibitors to produce EPDM with reduced branching. The ENB derived units are present in amounts well in excess of the amount of VNB. The spectrum of LCB and MWD variations that can be obtained appear to be limited by the process characteristics (a branching modifier is used). Very low levels of branching may be hard to obtain because of cationic branching generated by the ENB. Broad molecular weight distribution is favored.
In the present invention, an alternative method is used for controlling LCB, which permits greater reliance on the non-conjugated diene type which has two polymerizable double bonds, such as VNB. In this alternative method no, or much less, ENB can be used. Thus the benefits described for prior art EPDM polymers derived predominantly from VNB as the diene can be obtained, with the added benefit of balancing the influence of LCB on processing and the properties of the final product.
This method relies not on chemical branching modifiers, but on the predominant addition of the VNB (or equivalent diene having two polymerizable double bonds) in a second polymerization reaction step under polymerization conditions which allow for controlled incorporation of the VNB.
It is known to make EPDM type polyolefins, generally those having ENB derived units, in a continuous stirred tank series reactor layout, primarily to obtain broader molecular weight distributions and the attendant processability benefits resulting therefrom. Reference is made to U.S. Pat. No. 4,306,041; EP227206 and WO99/45047; WO99/45062 discusses polymer dispersions. The production of an EPDM product containing predominantly units of VNB for the diene so as to control levels of LCB is not described.
U.S. Pat. No. 6,319,998 and WO 99/45062 describe processes using metallocene type catalysts that have high activity and extremely efficient incorporation of diene. This leads to high levels of LCB, and in some cases the formation of gel. The process described herein employs a catalyst capable of controlling VNB incorporation so as to limit LCB formation.
For additional background see: WO 99/00434, U.S. Pat. No. 6,207,756, WO 98/02471, U.S. Pat. Nos. 3,674,754, 4,510,303 3,629,212, 4,016,342, 5,674,613, EP 1088855, U.S. Pat. No. 6,281,316, EP 784062, U.S. Pat. Nos. 4,510,303, 5,698,651 and 6,225,426.